[Issue 18564] New: core.demangle exception Range violation

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18564

          Issue ID: 18564
           Summary: core.demangle exception Range violation
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: johanengelen at weka.io

Testcase:
```
import core.demangle;
import std.stdio;

void main() {
    enum str =
"UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVVUUUUUUUUUUUUUYUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUYUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU";

    writeln(demangleType(str));
}
```

Instead of outputting the string, we get:
`core.exception.RangeError at core/demangle.d(230): Range violation`

(found by fuzz testing, but I get a range violation on the same line with a
real world type mangle too)

--